Exercise Makes You Crave Money Less

Our brains really do treat saving a penny and earning a penny as two completely different things.

A neat study just published in Medicine & Science in Sports & Exercise looks at the effect of aerobic exercise on the brain's reward system, using the chance of earning money as a reward. Researchers in Berlin had volunteers do either 30 minutes of treadmill exercise at 60-70% of VO2max, or 30 minutes of "placebo exercise" (light stretching, etc.). An hour later, they were put in a brain scanner and played a "monetary incentive delay" game, when they had to press buttons as quickly as possible in order to gain or avoid losing 1 euro at a time.

The main result is that, after aerobic exercise, the subjects had less activation of their reward systems in response to the prospect of gaining a euro. This result has been previously seen with other types of rewards like drugs and food, and probably relates to dopamine. Reward anticipation is associated with a short spike of dopamine that's quickly processed; but exercise causes a sustained rise of dopamine levels that lasts for a couple of hours. The result is the reward-linked spikes of dopamine are blunted during that post-exercise period.

Interesting side-note: exercise only had an effect on the brain's response to gaining a euro. There was no difference in how the brain responded to the prospect of avoiding the loss of a euro, even though it really amounts to the same thing. This was expected, and confirms the well-known fact that, despite Benjamin Franklin's advice, our brains really do treat saving a penny and earning a penny as two completely different things.

The experiment was performed with two very different groups of volunteers: one group was sedentary (less than one session of exercise per week), while the other group was made up of trained endurance athletes. Contrary to the expectations of the researchers, no differences were observed in the post-exercise reward response of the two groups. This was a bit of a surprise, and the researchers suggest that it may be because the exercise protocol (30 min at 60-70% VO2max) wasn't hard or long enough for the athletes. Still, it shows that even novice exercisers get a temporary suppression of reward activation after exercise. (That doesn't necessarily mean that they won't crave food: there are lots of competing chemical drivers of behavior in the body, and dopamine suppression may be trumped by other appetite-related factors.)